Both acrylate pressure sensitive adhesives and silicone pressure sensitive adhesives (PSAs) are known, and both have been found useful for a variety of purposes. Each, however, possesses certain disadvantages. For example, acrylate PSAs generally have poor low temperature flexibility and poor high temperature stability, and exhibit poor adhesion to low energy surfaces. Silicone PSAs, on the other hand, have excellent weatherability, are flexible at low temperature and stable at high temperatures, and exhibit excellent adhesion to low energy surfaces, but are costly, low in tack, and lacking in solvent resistance. Attempts have been made to provide "hybrid" systems having the advantages of each, but the approach generally taken has been to blend the two types of PSAs. Thus, these hybrids are prone to gross phase separation problems and their properties are also somewhat limited. In addition, the systems are solvent-based or water-based, necessitating a drying step.
For example, European Patent Publication No. 289928 (General Electric), published Nov. 9, 1988, describes an emulsion or solution comprising: (a) 100 parts by weight of water or organic solvent; (b) from about 10 to about 400 parts by weight of pressure sensitive adhesive comprising: (i) from about 50 to about 99% by weight organic pressure sensitive adhesive, preferably an acrylate, and (ii) from about 1 to about 50% by weight of silicone pressure sensitive adhesive; and (c) an effective amount of organic peroxide or alkoxy silane cross-linking agent to increase the shear strength of the composite adhesive through crosslinking of the silicone. The emulsion generally requires the use of an emulsifying agent or agents to maintain both the micelles of silicone adhesive and micelles of organic adhesive in a substantially stable state of suspension even at low water content, so that drying may be accomplished prior to gross phase separation of the silicone adhesive and the organic adhesive.
Similarly, U.S. Pat. No. 4,791,163 (Traver et al.) discloses an emulsion (formed from a silicone PSA and an organic PSA, preferably an acrylate) comprising: (a) 100 parts by weight of a continuous phase of water; (b) from about 10 to about 400 parts by weight of micelles comprising: (i) from about 50 to about 99% by weight of micelles comprising organic pressure sensitive adhesive, preferably an acrylate, and (ii) from about 1 to about 50% by weight of micelles comprising silicone pressure sensitive adhesive; and (c) an amount of emulsifying agent effective to maintain the emulsion. Curing of the silicone may be promoted by adding a peroxide or by adding a catalyst and an alkoxy silane.
Japanese Patent Publication No. 62-295982 (Toyota Gosei), published Dec. 23, 1987, describes organic solvent-based blends of silicone pressure sensitive adhesive, active hydrogen containing acrylic pressure sensitive adhesive, and polyurethane and/or polyisocyanate.
Japanese Patent Publication No. 60-197780 (Daicel), published Oct. 7, 1985, also discloses blends in organic solvent of 100 parts by weight acrylic pressure sensitive adhesive and 1-30 parts by weight silicone pressure sensitive adhesive.
Japanese Patent Publication Nos. 59-145269 (Nitto), published Aug. 20, 1984, and 63-291971 (Nitto), published Nov. 29, 1988, seek to avoid the gross phase separation problems characteristic of blends through the use of either bridging agents or compatibilizing agents. The former publication describes a composition comprising a medium, 100 parts by weight of acrylic adhesive polymer dissolved or dispersed in the medium, 5-120 parts by weight silicone adhesive polymer, and crosslinking agent capable of co-bridging both polymers. The latter publication discloses pressure sensitive adhesives comprising silicone pressure sensitive adhesive, polyacrylate pressure sensitive adhesive, and silicone polyacrylate graft copolymer.
A need thus exists for a hybrid PSA system which has the advantages of both acrylate PSAs and silicone PSAs which requires little or no solvent, thereby reducing or eliminating the environmental and health hazards associated with solvent use, as well as the need for drying. A need also exists for such a hybrid PSA system which is radiation curable and which, unlike most known hybrid systems, is not prone to gross phase separation problems. A need also exists for a hybrid PSA system which possesses balanced PSA properties tailorable over a wide range, thereby providing greater flexibility than known hybrid systems in achieving substrate-specific adhesion. We have discovered such a hybrid PSA system.